The landslide with locked flank in plane rotation is progressively formed while the slope rock mass shows an asymmetrical rotation revolving round on the axis perpendicular to the latent sliding plane. Its displacement mode may be generalized as a cantilever under both the well-distributed horizon load and the vertical compression. For the cantilever with a fracture situated on the locked later section where the maximum tensile stress appears, the elastic deflection-load equation is established by using Rayleigh-Ritz energy method. Then, the fracture criterion of the stress intensity factor is used to determine the critical load, and consequently the analytical formula of stability factor for the landslide is derived. In addition, a typical landslide is illustrated as an example of the use of above formula. Thus, the dynamics mechanism for the landslide is demonstrated theoretically.